With the incidence of heart failure steadily increasing the investigation into novel therapeutic interventions is in great demand. It is now understood that there is a direct correlation between the irreversible loss of cardiomyocytes and the progression of cardiac dysfunction and eventual failure. While both major types of cell death, apoptosis and necrosis, have distinct morphological features only apoptosis has been viewed as exhibiting a 'programmed signaling cascade.'This general dogma has left the understanding of necrosis and its role in many pathogenic states largely undefined. A primary event in mitochondrial-mediated cell death is the formation of a pore complex spanning the inner and outer mitochondrial membranes resulting in the loss of matrix and intermembrane contents. This process is known as mitochondrial permeability transition pore (MPTP) formation and is viewed as a primary mechanism of cell death resulting from a number of pathogenic stimuli prominent in heart failure. While the exact constituents of the MPTP remain unclear, the recent discovery that cyclophilin D is a master regulator of pore formation and seemingly indicative of necrotic, as opposed to apoptotic cell death, has opened the door allowing further understanding of these processes. Therefore, the current proposal seeks to test the following central hypothesis: Cyclophilin Dmediated MPTP formation and subsequent necrotic cell death is a primary mechanism driving the development and progression of heart failure. Specific aim 1 will determine the role of cyclophilin D utilizing gene-targeted mice in the progressive loss of cardiomyocytes and subsequent development of heart failure. In a translatinal approach, aim 2 will determine if pharmacologic inhibition of MPTP with the cyclophilin Dspecific inhibitor, DEBIO-025, reduces necrotic cell death in murine models of heart failure. The utilization of various gene-targeted mice coupled with the careful molecular evaluation of necrotic vs. apoptotic cell death will seek to define the contribution of necrosis in clinically relevant models of heart failure. Lay language: With heart failure quickly becoming an epidemic it is imperative that new therapies are discovered. This study seeks to understand the central role of cell death in the progression of heart failure. We seek to define specific signaling pathways that lead to the death of heart cells and the subsequent development of heart dysfunction. We will utilize the knowledge gained from this proposal in the application of novel drugs designed to target and interrupt cell death in the heart.